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Single Stage Peroxide Bleaching of ASAMBagasse Pulp .Part I: Effect of Some Bleaching Parameterson Pulp Properties .

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Shukry N, lEI-Kalyoubi SF, IHassan El-barbary M, 2Kordsacma 0,

ABSTRACT

The effect of some bleaching parameters on the properties of ASAMpulp from bagasse was studied. A chelation stage (Q)with DTPA priorto peroxide bleaching (P) is very useful for removing transition metalions that cause catalytic decomposition of the peroxide. Differentcharges of H] 0], MgS0

4and NaOH were used for bleaching in the

presence of DTPA or / and DTPMPA. Pulps having brightness above80% ISO could be obtained in a single peroxide bleaching treatmentusing 4% H]O], 2.5% NaOH, and 0.1-0.3% MgSO] in the presenceof 0.1% DTPMPA.

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Key words

H]O] bleaching, TCF bleaching, ASAM pulp, chelation, DTPA,DTPMPA,.

INTORDUCTION

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The demand for totally chlorine free TCF bleachedpulps has been rapidly growing during the last years.This indicates increased awareness and concern aboutenvironmental problems arising from the pulp andpaper industry. TCF bleaching sequences contribute todecreased pollution parameters such AOX, BOD, CODand colour and can eventually lead to totally effluentfree (TEF) pulp production (I). successful TCFbleaching requires a low kappa number of theunbleached pulp to start with because the commonchlorine free bleaching agents are less efficient thanchlorine {2,3}.

IThe present study aims at optimizing one-stage

H, 02 bleaching of bagasse pulp obtained by the ASAMprocess (alkaline sulfite pulping with addition of

IPPTA Vol. 12, No.3, Sept. 2000

anthraquinone and methanol). Unbleached ASAMpulps are characterized by very low kappa number athigh pulp yield and viscosity, which facilitate TCFbleaching. The necessity of a chelation stage prior toperoxide bleaching as well as the effects of somebleaching parameters, such as ~02 NaOH and MgS04

charges, and the type of chelant used within the Pstage were studied.

lCellulose and Paper Department,National Research Centre.Kokki, Cairo, Egypt.2Bundesforschungsanstalt fur Forst-undHolzwirtschaft, Institut fur Holzchemie undehemische technologie des Holzes, Leuschnerstr.91, D-21031 Hamburg

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EXPERIMENTAL

Pulping

Bagasse was pulped as follows: chemical charge(as NaOH o.d. bagasse) =16%, alkali ratio (Na,SO/total alkali) =0.7, AQ (o.d.bagasse) =0.1%, methanolcharge (on total liquor, v/v) = 15%, liquor ratio = 4: I,heat up time 45 min, cooking time at maximumtemperature of 170°£ = 60 min. Pulping was conductedwith charges of 600 g o.d. bagasse in a rotatingautoclave at the Institute of Wood Chemistry, Universityof Hamburg. Germany. The pulping conditions appliedrepresent the optimum conditions obtained after a seriesof experiments carried out at our laboratory in Egypt.Our experiments were performed on a small scale with100 g o.d. bagasse. The small autoclaves were rotatingin a temperature controlled oil bath. Different chemicalcharges (14,16,18,20%), alkali ratios (0.6,0.7,0.8), andmethanol concentrations (0.5,10,15, 20, 25, 30, 35%vrv of liquor ) were investigated applying differentcooking times (30-170min) at 165°C.

PULP PRETREATMENTA chelation stage with DTPA

(diethylenetriantinepentaacetic acid) was carried out toremove transition metals ions. The pulp was mixedwith the amount of deionized water of 6,00Cnecessaryto adjust 3% pulp consistency. The pulp slurry wasacidified to pH 4.5 -5.5 before adding the chelant (0.2%DTPA based on o.d. pulp). After 30 min at 60°C, thepulp suspension was washed on a Buchner funnel,pressed and centrifuged to about 30-35% consistency.

BLEACHING EXPERllWENTSThe peroxide stage was carried out for 120 min at

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800C and 10% consistency. The chemical chargesapplied are given in the eorresponding tables. Theorder of addition of the chemicals was standardized asfollows: First MgS04 with chelant, then NaOH andfinally ~02' At first the pulp was placed in a plasticbag and the chemicals were added. The pulp waskneaded by hand and the initial pH measured. Theplastic bag was then sealed and immersed in a waterbath. At the end of the bleaching, the plastic bag wascooled and the liquid was squeezed from the pulp formeasuring final pH and residual peroxide. The bleachedpulp was then washed throughly with dionized waterand centrifuged to 30-35% consistency. The residualperoxide in the filtrate was determined iodometrically.

ANALYSIS OF THE PULPTappi 236 - em for the unbleachedpulp and Tappi UM 246 for bleachedpulp

ISO units measured on pulp sheetsprepared according to Scan ~1.75

modified method bas{d' onZellcheming Merkblatt IV 136/61 and,Scan -CM 15:88

RESUL TS AND DISCUSSION

Kappa number

Brightness

Viscosity

Effect of chelation (Q stage) before peroxidebleaching

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A chelation stage with DTPA was performed onfour unbleached ASAM pulps Table 1 shows transitionmetal ions content, brightness, kappa number andviscosity of the pulps before (B) and after thispretreatment (Q). The amount of Mn, Mg, Cu and Fe

TABLE 1. Metal ions content and properties of four ASAM bagasse pulps before (B) and afterchelatiol\~th 0.2 % ntpA (Q)

Sample Mn Mg Cu Fe Brightness Brightness Kappa Viscosity

ppm ppm ppm ppm (% ISO) gain number ml/g(% ISO)

B1 15 205 57 129 51.4 4.9 789

Q1 5.0 141 5.7 111 59.3 7.9 4.5 801 •B2 10 215 23 110 49.9 4.7 823

Q2 5.0 154 5.7 luI 54.3 4.4 4.2 837

B3 10 208 29 128 48.7 4.8 795

Q3 5.0 150 5.8 100 53.2 4.5 4.4 797

B4 9.9 341 54 198 56.8 5.2 856

~ ~~. 5.0 191 9.2 164 61.8 5.0 4.6 819

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TABLE 2. Results of peroxide stages carried out on pretreated (Pt,l,3.) and untreated(P, •.2-, J•••• ) ASAM Pulps.

Sample MgSO. DTPA DTPMPA Kappa Residual ~02 Brightness Brightness% 0/0 o/. number (% of charge) (~o ISO) gain

(~oISO)

BI - - - 4.9 - 51.3

QI. - - - 4.5 - 59.3 8.0

PI 0.1 0 0.1 2.3 17.8 81.1 29.8

p* 0.1 0 0.1 2.3 0.4 74.2 22.9IP2 0.3 0 0.1 2.3 24.9 80.3 29p* 0.3 0 0.1 2.4 0.3 73.6 22.3

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P3 0.3 0.1 0.2 2.3 32.4 80.9 29.6p. 0.3 0.1 0.2 2.4 1.9 76.3 25.0

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p.• 0.3 0.2 0.4 2.4 46.5 81.5 30.2p* 0.3 0.2 0.4 2.4 10.9 78.4 27.1

4'>S-

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Bleaching conditions: Temperature = 80DC, Time = 120 min. ~02 = 4~o, NaOB=2.5%

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ions. in the pulps before and after chelation weredetermined by AAS. About 60% of Mn ions. 40% ofMg ions. 8S%·of Cu ions. and 15% of Fe ions wereremoved from the pulp by the DTPA treatment.Brightness gains ranging from 4.5 - 7.9 ISO unitswere obtained after the Q stage slight drops in thekappa number and times in the viscosity were attained.

The necessity of a chelation stage before bleachingcould be proven by peroxide bleaching tests onpertreated and untreated pulp samples. DTPA and lorDTPMPA (diethylenetriaminepentamethylene-phosphonic acid) were added in the P stage (Tab.2).As a result of partly removing the harmful metal ionsin the Q stage, the peroxide decomposition was

restricted and the pulp brightness was significantlyimproved compared to peroxide bleaching withoutpretreatment. The use of a chelan! oll--eventwo chelantsonly with the P stage was not sufficient. Even at highcharges. the chelants were not able to stabilize theperoxide. The much lower brightness gain achievedindicates excessive catalytic decomposition of theperoxide.

EFFECT OF ~02 CHARGE AND NaOH: H202RATIO ON PULP PROPERTIES

It is obvious from Table-3 that 1% ~02 is notsuffficient to delignify and brighten the pulp. Byincreasing the charge of ~02 to 2% the delignification

TABLE 3. Effect of different ~02 and NaOB charges ~ASA.M pulp properties

Sample B202 NaOB Residual B202 Kappa Viscosity Brightness Brightness0/0 ./. (./. of charge) number (mllg) (o/. ISO) gain

(%ISO)

QI - - - 4.5 801 59.3 -Ps 1 1.75 3.4 3.1 775 69.1 9.8P

6. 2 2.0 8.1 2.7 751 73.1 13.8

Po ~,j .," 2.25 16.2 2.6 768 76.2 16.97 "'. Pa '.' .,t ~.S4'>- 22.3 2.6 761 78.7 19.4-, f '.

P9 5 3.0· 5.1 2.3 728 79.3 20.0PIO 6 3.5 3.68 2.2 716 78.9 19.6

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Bleaching conditions: Temperature = 80°C, Time = 120 min, MgSO .•= 0.3~0, DTPA = 0.1%

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TABLE 4. Effect of NaOH charge at constant H101 charge on ASAM pulp properties.

Sample ~Ol NaOH pH Residual H101 Brightness Brightness0/0 0/0 initial final (% of charge) (% ISO) gain

(OfoISO)

Pn 4 1.5 11.6 11.8 48.9 76.6 17.3

PI24 2.0 11.5 11.8 20.2 77.1 17.8

Ps 4 2.5 11.3 11.7 22.3 78.7 19.4

PI34 3.0 11.3 11.5 6.4 76.4 17.1

PI44 3.5 11.3 11.5 5.9 75.8 16.5

Bleaching conditions: Temperature = 80°C, Time = 120 min, MgSO. = 0.3%, DTPA = 0.1%

TABLE 5. Effect of MgS04

charge at constant H101 charge on ASAM pulp properties.

Sample MgSO. DTPA DTPMPA Kappa pH Residual H101 Brightness Brightness0/0 0/0

% number (% of charge) (0/. ISO) gaininitial final (o/.ISO)

QI - - 4.5 - - - 59.3 --PIS 0 0 0 2.4 11.3 12.2 0.2 77.4 . 18.1

PI6 0 0.1 0 2.4 11.3 12.3 0.1 76.5 17.2

P17 0 0 0.1 2.2 11.4 12.2 4.7 80.7 21.4

PI 0.1 0 0.1 2.3 11.3 12.0 17.8 81.1 20.8

PIS 0.2 0 0.1 2.3 11.4 12.0 20.8 80.5 21.2

Pl 0.3 0 0.1 2.3 11.3 11.7 24.9 80.3 21.0

PI9 0.5 0 0.1 2.3 11.3 11.6 33.2 75.6 163

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Bleaching conditions: Temperature = 80°C, Time = 120 min, Hl01 = 4%, NaOH = 2.5%

and brightening processes were considerably improved.Further increase of the peroxide charge up to 4%improved only the brightness, whereas the kappanumber was not further reduced at higher charges ofhydrogen peroxide (5-6%) the carbohydrates wereattached more intensively. The drop in the viscositycaused by raising the peroxide charge from 4 to 6%was 45 ml/g, whereas the drop in viscosity affected byincreasing the peroxide from 1 to 4% was only14 ml/g. Therefore, 4% H202 was considered as theoptimum peroxide charge.

Table 4 and figure 1 show the effect of increasingthe NaOH charge at 4% HP2 on the properties of theASAM bagasse pulp. Up to a charge of 2.5 % (0.625NaOH: HP) the brightness increases steadily. A higheralkali charge, however, leads to a too high peroxideconsumption and the brightening effect decreases dueto alkaline darkening.

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EFFECT OF MgS04 AND THE CHELANTSON THE STABILIZATION OF n,o,

The amount of MgS04 added to the bleach systemwas varied, while the peroxide and NaOH charges werekept constant at 4 and 2.5% respectively. Table-S showsthe results.

Experiment no.15 was carried out without additionof any stabilizer. The detrimental effect on thestabilization of H202 is very obvious, the peroxide wasalmost totally consumed. Addition of 0.1 % DTPA didnot improve the bleaching result (experiment no.16).The use of 0.1% DTPMPA instead ofDTPA (experimentno.17) resulted in a somewhat better peroxidestabilization and an increased brightness. However,the rather low amount of residual peroxide indicates anot optimal bleach performance, since the H202 residual

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0.375 0.5 0.625 0.75 0.875NaOH : H202 MgS04,%

Figure 1: Effect of NaOH: H10

2ratio on Figure 2: Effect of MgS04 charge on residual

brightness gain and residual peroxide of ASAM peroxide of ASAM pulp.TABLE 6. Bleach results obtained by using DTPA and DTPMP A in the P stage.

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- Brightness galn(%150)--6- Residual peroxide, %

20 .-----------, 60

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40~------------------------~iI!! 36~' 30.;ce 25~ 20-

~ 15"C·iii 10~ 5O~--~--~----~--~--~--~

o 0.1 0.2 0.3 0.4 0.5 0.6

S•• ple MgS04 DTPA DTPMPA pH Kapp. Residual ~02 Brightoess gaio0/0 O/e e/o ou.ber (e/. of charge) ISO uoits

ioitial fioal (%ISO)

P:zo 0.1 0.1 0 11.3 12.5 2.4 0.5 20.2

P110.2 0.1 0 11.4 12.5 2.3 1.8 19.1

p. 0.3 0.1 0 11.3 11.7 2.6 22. 19.5

PI 0.1 0 0.1 11.3 12.0 2.4 17. 21.9

PIS 0.2 0 0.1 11.4 12.0 2.3 20.8 21.2

P10.3 0 0.1 11.3 11.7 2.3 24.9 21.00

P, 0.3 0.1 0.2 11.3 11.7 2.3 32.4 21.7

P40.3 0.2 0.4 11.3 11.3 2.4 46.5 22.3

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Bleaching conditions: Temperature = 80°C, Time =< 120 min, H201 = 4~/e,NaOH == 2.S%.

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should not be lower than 15-20 {4}. By introducingMgS0

4to the bleaching system (experiments 1,18,2,19)

a substantially improved stabilization of the peroxidewas observed, as expressed by the increassed amountof residual H202• The extent of stabilization wasproportional to the MgS04 charge (Fig.2). In case ofabsence of MgS04 the amount of residual peroxide was4.7%. With increase ofthe MgS04 charge from 0.1 to0.5 the residual peroxide increased from 17.8 to 33.2%.This result confirms the important role of MgS04 indeactivating the transition inetals stitt present in thebleaching system (5). It was also found that the presenceof MgS0

4in amounts ranging from 0.1-0.3 (based on

o.d. pulp) is beneficial. but an increase of the charge to0.5% had an adverse effect on the brightness gain,which indicates that the brightening effect is suppressedby a too high amount of Mg. Our results are inaccordance with those obtained by Desprez et at. {6}and van Lierop et al {7}. As given in Tab. 5 the initial

IPPTA Vol. 12, No.3, Sept. 2000

pH of the bleaching system was 11.3 and the final pHwas higher, which means that during bleaching the pHincreases due to the consumption of peroxide. Such anincrease of the pH is favourable in some cases, becausefinal pH of 11.7 to 12 can be considered as optimalwith respect to peroxide decomposition reactions. Itcan also be concluded that the final pH should notexceed 12, otherwise the peroxide decomposition isaccelerated (cf. experiments 15-17).

EFFECT OF THE TYPE OF CHELANTADDED IN THE P STAGE ON PULPPROPERTIES

In another set of experiments it was tested ifDTPA or DTPMPA is more appropriate to stabilize theperoxide in combination MgS04 added in diferentquaties (Tab.6).

At a DTPA charge of 0.1% based on o.d. pulp, an

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increased charge of MgS04 from 0.1 to 0.3 resulted inan increase of the residual peroxide from 0.5 to 22.3%,but the better peroxide stability was not associatedwith an improved brightening effect (experiments20,21,8). On the other hand DTPMPA stabilized theperoxide very efficiently even at low MgS04 chargeand a better bleaching effect was obtained (experiments1,18,2). It can be concluded that, applied with the Pstage, DTPMP A is superior to DTPA, especially at lowMgS04 charges. The use of both chelants in differentquantities enables an even better peroxide stabilization

i but the achieved improvement in brightness was'negligible (experiments .3 and 4). From the economicpoint of view it is advisable to use 0.1% DTPMPAtogether with 0.1% MgS04 for peroxide stabilization.

CONCLUSION

ASAM pulping of bagasse results in easilybleachable pulps with low kappa number and highinitial brightness. A single P stage is sufficient to reacha target brightness of 80% ISO. However, a precedingchelation stage is necessary to remove transition metalions which catalyze the decomposition of ~02 in thesubsequent P stage. The importance of this pretreatmentcould be proven by comparison with reference bleachingtests performed with the untreated pulp. Thepretreatment resulted in higher peroxide stability andin significantly higher pulp brightness. It was alsofound that charges of 4% ~02 and 2.5% NaOH (basedon o.d. pulp) give optimum results. Moreover, the

, presence of MgS04 and a chelant in the bleach systemis a must in order to stabilize the peroxide. However,too high MgS04 charges (>0,3%) suppress the bleachingprocess. DTPA stabilizes the system only at highMgS04 charges, whereas DTPMPA shows a goodstabilizing effect even at a low MgS04 charge.

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ACKNOWLEDGEMENT

Part of this work was carried out at the Instituteof wood Chemistry and Chemical Technology of Wood,University of Hamburg, Germany. The authors thankProf. Dr. R. Patt for his encouragement and helpfuladvises.

REFERENCES

1. Malinen R, Fuhrmann A. Recent trends inbleaching of chemical pulp. Paperi Ja Puu 1995:77 (3): 78-83.

2. Korhonen R. TCF and the totally-closed cycle.PPI 1993 : June: 57-59

3. Brooks TR, Edwards LL. Nepote ~C. CaldwellMR. Bleach plant closeup and conversion toTCF : a case study using mill data and computersimulation. Tappi J. 1994 : 77 (11) : 83-92.

4. Patt R. Private communication (1997).

5. Sjostrom E, Vattila 0. Inhibition of carbohydratedegradation during oxygen bleaching. 1.Comparison of various additives. Paperi Ja Puu1972 : 54 (11): 695-700.

6. Desprez F, Devenyns J, Troughton NA. TCF fullbleaching of softwood kraft pulp: The optimalconditions for p* hydrogen peroxide bleaching.Tappi Pulping Conference. San Diego. 1994:929-934.

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7. Van Lierop B, Liebergott N. Faubert MG. Usingoxygen and peroxide to bleach kraft pulps. J. ofPulp & Paper Sci. 1994: 20 (7): 1193-1198.

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IPPTA Vol. 12, No.3. Sept. 2000